Internet small computer interface systems extension for remote direct memory access (rdma) for distributed hyper-converged storage systems

ABSTRACT

Certain Embodiments described herein relate to configuring the network-storage stack of two devices (e.g., physical or virtual) communicating together (e.g., an initiator and a target, as defined below) with Internet Small Computer Systems Interface (iSCSI) extension for remote direct memory access (RDMA) iSER, which is a protocol designed to utilize RDMA to accelerate iSCSI data transfer. The iSER protocol is implemented as an iSER datamover layer that acts as an interface between an iSCSI layer and an RDMA layer of the network-storage stacks of the two devices. Using iSER in conjunction with RDMA allows for bypassing the existing traditional network protocol layers (e.g., TCP/IP protocol layers) of the devices and permits data to be transferred directly, between the two devices, using certain memory buffers, thereby avoiding memory copies taking place when the existing network protocol layers are used.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to International Application No.PCT/CN2019/106151, filed Sep. 17, 2019. The content of the applicationis hereby incorporated by reference in its entirety.

BACKGROUND

Distributed systems allow multiple clients in a network to access a poolof shared resources. For example, a distributed storage system allows acluster of host computers or other computing systems (“nodes”) toaggregate local storage devices (e.g., SSD, PCI-based flash storage,SATA, or SAS magnetic disks) located in or attached to each node tocreate a single and shared pool of storage. This pool of storage(sometimes referred to herein as a “datastore” or “store”) is accessibleby all nodes in the cluster and may be presented as a single namespaceof storage entities (such as a hierarchical file system namespace in thecase of files, a flat namespace of unique identifiers in the case ofobjects, etc.). Storage clients in turn, such as virtual computinginstances (VCIs) (e.g., virtual machines (VMs), containers, etc.)spawned on host computers or physical machines may use the datastore tostore data. In one example, virtual machines may use the datastore tostore virtual disks that are accessed by the virtual machines duringtheir operation. The virtual disks may be stored in the datastore in theform of objects, which may also be referred to as virtual disk objects.Nodes in the cluster may access virtual disk objects stored in othernodes in the cluster using a protocol referred to as Small ComputerSystems Interface (SCSI), which comprises a set of interfaces that allownodes in the cluster to access storage resource of other nodes in thecluster.

In some cases, to make the data, such as virtual disk objects, availableto computing systems (e.g., physical or virtual) outside of the clusterof nodes, each node in the cluster may further be configured with theInternet Small Computer Systems Interface (iSCSI). iSCSI, is an InternetProtocol (IP)-based storage networking standard for linking the nodes inthe cluster to the nodes or workloads outside of the distributed storagesystem. Generally, iSCSI is implemented as a protocol layer to interactwith the Transmission Control Protocol (TCP) protocol layer in a networkstack of a node within the cluster, thereby, enabling the node toexchange SCSI commands with a node outside the cluster over a network,such as a layer-3 network. However, using the TCP protocol layer mayresult in low input/output (I/O) performance and high central processingunit (CPU) utilization.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example computing environment in which one or moreembodiments may be implemented, according to certain embodiments.

FIG. 2 illustrates an example hierarchical structure of objectsorganized within an object store that represent a virtual disk,according to certain embodiments.

FIG. 3 illustrates components of a virtual storage area network moduleimplemented in the computing environment of FIG. 1, according to certainembodiments.

FIG. 4 illustrates an example network-storage protocol stack, accordingto certain embodiments.

FIG. 5 illustrates an example network-storage protocol stack with aniSCSI extension for RDMA (iSER), according to certain embodiments.

FIG. 6 illustrates an example connection lifecycle management procedurebetween an iSER target and an iSER initiator, according to certainembodiments.

FIG. 7 illustrates example operations performed by a network-storagestack at an iSER target for processing an incoming I/O request in theform of an iSER packet, from an iSER initiator, according to certainembodiments.

FIG. 8 illustrates an example network-storage protocol stack with aniSER, according to certain embodiments.

FIG. 9 illustrates operations performed by network-storage stack at aniSER target for processing an incoming I/O write request in the form ofan iSER packet, from an iSER initiator.

DETAILED DESCRIPTION

Embodiments described herein relate to configuring the network-storagestack of two devices (e.g., physical or virtual) communicating together(e.g., an initiator and a target, as defined below) with iSER, which isa protocol designed to utilize RDMA to accelerate iSCSI data transfer.The iSER protocol is implemented as an iSER datamover layer that acts asan interface between an iSCSI layer and an RDMA layer of thenetwork-storage stacks of the two devices. Using iSER in conjunctionwith RDMA allows for bypassing the existing traditional network protocollayers (e.g., TCP/IP protocol layers) of the devices and permits data tobe transferred directly, between the two devices, using certain memorybuffers, thereby avoiding memory copies taking place when the existingnetwork protocol layers are used.

FIG. 1 illustrates an example computing environment in which one or moreembodiments may be implemented. As shown, computing environment 100 is asoftware-based “virtual storage area network” (VSAN) environment thatleverages the commodity local storage housed in or directly attached(hereinafter, use of the term “housed” or “housed in” may be used toencompass both housed in or otherwise directly attached) to hostservers, or nodes 111 of a cluster 110, to provide an aggregate objectstore 116 to virtual machines (VMs) 112 running on nodes 111. The localcommodity storage housed in or otherwise directly attached to each node111 may include combinations of solid state drives (SSDs) 117 and/ormagnetic or spinning disks 118. In certain embodiments, SSDs 117 serveas a read cache and/or write buffer in front of magnetic disks 118 toincrease I/O performance.

In addition, as further discussed below, each node 111 may include astorage management module (referred to herein as a “VSAN module”) inorder to automate storage management workflows (e.g., create objects inthe object store, etc.) and provide access to objects in the objectstore (e.g., handle I/O operations to objects in the object store, etc.)based on predefined storage policies specified for objects in the objectstore. For example, because a VM may be initially configured by anadministrator to have specific storage requirements for its “virtualdisk” depending on its intended use (e.g., capacity, availability, IOPS,etc.), the administrator may define a storage profile or policy for eachVM specifying such availability, capacity, IOPS and the like. As furtherdescribed below, the VSAN module may then create an “object” for thespecified virtual disk by backing it with the datastore of the objectstore based on the defined policy.

A virtualization management platform 105 is associated with cluster 110of nodes 111. Virtualization management platform 105 enables anadministrator to manage the configuration and spawning of VMs on thevarious nodes 111. As depicted in the embodiment of FIG. 1, each node111 includes a virtualization layer or hypervisor 113, a VSAN module114, and hardware 119 (which includes conventional computing hardware,such as one or more central processing units, random access memory,read-only memory, one or more network interface controllers, SSDs 117,and magnetic disks 118). Through hypervisor 113, a node 111 is able tolaunch and run multiple VMs 112. Hypervisor 113, in part, manageshardware 119 to properly allocate computing resources (e.g., processingpower, random access memory, etc.) for each VM 112. Furthermore, asdescribed further below, each hypervisor 113, through its correspondingVSAN module 114, provides access to storage resources located inhardware 119 for use as storage for virtual disks (or portions thereof)and other related files that may be accessed by any VM 112 residing inany of nodes 111 in cluster 110.

In one embodiment, VSAN module 114 is implemented as a “VSAN” devicedriver within hypervisor 113. VSAN module 114 provides access to aconceptual VSAN 115 through which an administrator can create a numberof top-level “device” or namespace objects that are backed by objectstore 116. In one common scenario, during creation of a device object,the administrator specifies a particular file system for the deviceobject (such device objects hereinafter also thus referred to “filesystem objects”). For example, each hypervisor 113 in each node 111 may,during a boot process, discover a /vsan/ root node for a conceptualglobal namespace that is exposed by VSAN module 114. By accessing APIsexposed by VSAN module 114, hypervisor 113 can then determine all thetop-level file system objects (or other types of top-level deviceobjects) currently residing in VSAN 115. When a VM (or other client)attempts to access one of the file system objects, hypervisor 113 maydynamically “auto-mount” the file system object at that time. In certainembodiments, file system objects may further be periodically“auto-unmounted” when access to objects in the file system objects ceaseor are idle for a period of time. A file system object (e.g.,/vsan/fs_name1, etc.) that is accessible through VSAN 115 may, forexample, be implemented to emulate the semantics of a particular filesystem such as a virtual machine file system, VMFS, which is designed toprovide concurrency control among simultaneously accessing VMs. BecauseVSAN 115 supports multiple file system objects, it is able to providestorage resources through object store 116 without being confined bylimitations of any particular clustered file system. For example, manyclustered file systems (e.g., VMFS, etc.) can only scale to support acertain amount of nodes 111. By providing multiple top-level file systemobject support, VSAN 115 overcomes the scalability limitations of suchclustered file systems.

A file system object, may, itself, provide access to a number of virtualdisk descriptor files accessible by VMs 112 running in cluster 110.These virtual disk descriptor files contain references to virtual disk“objects” that contain the actual data for the virtual disk and areseparately backed by object store 116. A virtual disk object may itselfbe a hierarchical or “composite” object that, as described furtherbelow, is further composed of “component” objects (again separatelybacked by object store 116) that reflect the storage requirements (e.g.,capacity, availability, IOPs, etc.) of a corresponding storage profileor policy generated by the administrator when initially creating thevirtual disk. Each VSAN module 114 (through a cluster level objectmanagement or “CLOM” sub-module) communicates with other VSAN modules114 of other nodes 111 to create and maintain an in-memory metadatadatabase (e.g., maintained separately but in synchronized fashion in thememory of each node 111) that contains metadata describing thelocations, configurations, policies and relationships among the variousobjects stored in object store 116. This in-memory metadata database isutilized by a VSAN module 114 on a node 111, for example, when anadministrator first creates a virtual disk for a VM as well as when theVM is running and performing I/O operations (e.g., read or write) on thevirtual disk. As further discussed below in the context of FIG. 3, VSANmodule 114 (through a document object manager or “DOM” sub-module, insome embodiments as further described below) traverses a hierarchy ofobjects using the metadata in the in-memory database in order toproperly route an I/O operation request to the node that houses theactual physical local storage that backs the portion of the virtual diskthat is subject to the I/O operation.

FIG. 2 illustrates an example hierarchical structure of objectsorganized within object store 116 that represent a virtual disk. Aspreviously discussed above, a VM 112 running on one of nodes 111 mayperform I/O operations on a virtual disk that is stored as ahierarchical or composite object 200 in object store 116. Hypervisor 113provides VM 112 access to the virtual disk by interfacing with theabstraction of VSAN 115 through VSAN module 114 (e.g., by auto-mountingthe top-level file system object corresponding to the virtual diskobject, as previously discussed). For example, VSAN module 114, byquerying its local copy of the in-memory metadata database, is able toidentify a particular file system object 205 (e.g., a VMFS file systemobject) stored in VSAN 115 that stores a descriptor file 210 for thevirtual disk. It should be recognized that the file system object 205may store a variety of other files consistent with its purpose, such asvirtual machine configuration files and the like when supporting avirtualization environment. In certain embodiments, each file systemobject may be configured to support only those virtual diskscorresponding to a particular VM (e.g., a “per-VM” file system object).

Descriptor file 210 includes a reference to composite object 200 that isseparately stored in object store 116 and conceptually represents thevirtual disk (and thus may also be sometimes referenced herein as avirtual disk object). Composite object 200 stores metadata describing astorage organization or configuration for the virtual disk (sometimesreferred to herein as a virtual disk “blueprint”) that suits the storagerequirements or service level agreements (SLAs) in a correspondingstorage profile or policy (e.g., capacity, availability, IOPs, etc.)generated by an administrator when creating the virtual disk. Forexample, in the embodiment of FIG. 2, composite object 200 includes avirtual disk blueprint 215 that describes a RAID 1 configuration wheretwo mirrored copies of the virtual disk (e.g., mirrors) are each furtherstriped and partitioned in a RAID 0 configuration. Composite object 200may thus contain references to a number of “leaf” or “component” objects220 _(x) corresponding to each data chunk (e.g., data partition of thevirtual disk) in each of the virtual disk mirrors. The metadataaccessible by VSAN module 114 in the in-memory metadata database foreach component object 220 (e.g., for each stripe) provides a mapping toor otherwise identifies a particular node 111 _(x) in cluster 110 thathouses the physical storage resources (e.g., SDD 117, magnetic disks118, etc.) that actually stores the data chunk (as well as the locationof the data chunk within such physical resource). The RAID 1/RAID 0combination is merely an example of how data associated with a compositeobject 200 may be stored by nodes 111 (e.g., nodes 111 a-111 f) of nodecluster 110. In other examples, all data associated with compositeobject 200 may be stored in one node (e.g., node 111 a). In yet anotherexample, data associated with composite object 200 may be only mirroredby a RAID 1 operation such that one copy of the data may be stored inone node and another copy may be stored by another node. In otherexamples, other RAID operations or a combination of a variety of RAIDoperations (e.g., RAID1/RAID 5) may be used when distributing dataassociated with composite object 200. Regardless of how data associatedwith a VM 112's virtual disk is partitioned or copied across nodes,however, the data is still stored as a plurality of data blocks.

FIG. 3 illustrates components of VSAN module 114. As previouslydescribed, in certain embodiments, VSAN module 114 may execute as adevice driver exposing an abstraction of a VSAN 115 to hypervisor 113.Various sub-modules of VSAN module 114 handle different responsibilitiesand may operate within either user space 315 or kernel space 320depending on such responsibilities. As depicted in the embodiment ofFIG. 3, VSAN module 114 includes a cluster level object management(CLOM) sub-module 325 that operates in user space 315. CLOM sub-module325 generates virtual disk blueprints during creation of a virtual diskby an administrator and ensures that objects created for such virtualdisk blueprints are configured to meet storage profile or policyrequirements set by the administrator. In addition to being accessedduring object creation (e.g., for virtual disks), CLOM sub-module 325may also be accessed (e.g., to dynamically revise or otherwise update avirtual disk blueprint or the mappings of the virtual disk blueprint toactual physical storage in object store 116) on a change made by anadministrator to the storage profile or policy relating to an object orwhen changes to the cluster or workload result in an object being out ofcompliance with a current storage profile or policy.

In some embodiments, if an administrator creates a storage profile orpolicy for a composite object such as virtual disk object 200, CLOMsub-module 325 applies a variety of heuristics and/or distributedalgorithms to generate virtual disk blueprint 215 that describes aconfiguration in cluster 110 that meets or otherwise suits the storagepolicy (e.g., RAID configuration to achieve desired redundancy throughmirroring and access performance through striping, which nodes' localstorage should store certain portions/partitions/stripes of the virtualdisk to achieve load balancing, etc.). For example, CLOM sub-module 325,in some embodiments, is responsible for generating blueprint 215describing the RAID 1/RAID 0 configuration for virtual disk object 200in FIG. 2 when the virtual disk was first created by the administrator.As previously discussed, a storage policy may specify requirements forcapacity, IOPS, availability, and reliability. Storage policies may alsospecify a workload characterization (e.g., random or sequential access,I/O request size, cache size, expected cache hit ratio, etc.).Additionally, the administrator may also specify an affinity to VSANmodule 114 to preferentially use certain nodes 111 (or the local diskshoused therein). For example, when provisioning a new virtual disk for aVM, an administrator may generate a storage policy or profile for thevirtual disk specifying that the virtual disk have a reserve capacity of400 GB, a reservation of 150 read IOPS, a reservation of 300 write IOPS,and a desired availability of 99.99%. Upon receipt of the generatedstorage policy, CLOM sub-module 325 consults the in-memory metadatadatabase maintained by its VSAN module 114 to determine the currentstate of cluster 110 in order generate a virtual disk blueprint for acomposite object (e.g., the virtual disk object) that suits thegenerated storage policy. As further discussed below, CLOM sub-module325 may then communicate the blueprint to its corresponding distributedobject manager (DOM) sub-module 340 which interacts with object space116 to implement the blueprint by, for example, allocating or otherwisemapping component objects (e.g., stripes) of the composite object, andmore particularly, data blocks of component objects, to physical storagelocations within various nodes 111 of cluster 110.

In addition to CLOM sub-module 325 and DOM sub-module 340, as furtherdepicted in FIG. 3, VSAN module 114 may also include a clustermonitoring, membership, and directory services (CMMDS) sub-module 335that maintains the previously discussed in-memory metadata database toprovide information on the state of cluster 110 to other sub-modules ofVSAN module 114 and also tracks the general “health” of cluster 110 bymonitoring the status, accessibility, and visibility of each node 111 incluster 110. The in-memory metadata database serves as a directoryservice that maintains a physical inventory of the VSAN environment,such as the various nodes 111, the storage resources in the nodes 111(SSD, magnetic disks, etc.) housed therein and thecharacteristics/capabilities thereof, the current state of the nodes 111and their corresponding storage resources, network paths among the nodes111, and the like. As previously discussed, in addition to maintaining aphysical inventory, the in-memory metadata database further provides acatalog of metadata for objects stored in object store 116 (e.g., whatcomposite and component objects exist, what component objects belong towhat composite objects, which nodes serve as “coordinators” or “owners”that control access to which objects, quality of service requirementsfor each object, object configurations, the mapping of objects tophysical storage locations, etc.). As previously discussed, othersub-modules within VSAN module 114 may access CMMDS sub-module 335(represented by the connecting lines in FIG. 3) for updates to learn ofchanges in cluster topology and object configurations. For example, aspreviously discussed, during virtual disk creation, CLOM sub-module 325accesses the in-memory metadata database to generate a virtual diskblueprint, and in order to handle an I/O operation from a running VM112, DOM sub-module 340 accesses the in-memory metadata database todetermine the nodes 111 that store the component objects (e.g., stripes)of a corresponding composite object (e.g., virtual disk object) and thepaths by which those nodes are reachable in order to satisfy the I/Ooperation.

As previously discussed, during the handling of I/O operations as wellas during object creation, DOM sub-module 340 controls access to andhandles operations on those component objects in object store 116 thatare stored in the local storage of the particular node 111 in which DOMsub-module 340 runs as well as certain other composite objects for whichits node 111 has been currently designated as the “coordinator” or“owner.” For example, when handling an I/O operation from a VM, due tothe hierarchical nature of composite objects in certain embodiments, aDOM sub-module 340 that serves as the coordinator for the targetcomposite object (e.g., the virtual disk object that is subject to theI/O operation) may need to further communicate across the network with adifferent DOM sub-module 340 in a second node that serves as thecoordinator for the particular component object (e.g., data chunk, etc.)of the virtual disk object that is stored in the local storage of thesecond node 111 and which is the portion of the virtual disk that issubject to the I/O operation. If the VM issuing the I/O operationresides on a node 111 that is also different from the coordinator of thevirtual disk object, the DOM sub-module 340 of node 111 running the VMwould also have to communicate across the network with the DOMsub-module 340 of the coordinator. In certain embodiments, if the VMissuing the I/O operation resides on a node that is different from thecoordinator of the virtual disk object subject to the I/O operation, thetwo DOM sub-modules 340 of the two nodes may need to communicate tochange the role of the coordinator of the virtual disk object to thenode running the VM (e.g., thereby reducing the amount of networkcommunication needed to coordinate I/O operations between the noderunning the VM and the node serving as the coordinator for the virtualdisk object).

DOM sub-modules 340 also similarly communicate amongst one anotherduring object creation. For example, a virtual disk blueprint generatedby CLOM module 325 during creation of a virtual disk may includeinformation that designates which node 111 should serve as thecoordinators for the virtual disk object as well as its correspondingcomponent objects (stripes, etc.). Each of the DOM sub-modules 340 forsuch designated nodes is issued requests (e.g., by the DOM sub-module340 designated as the coordinator for the virtual disk object or by theDOM sub-module 340 of the node generating the virtual disk blueprint,etc. depending on embodiments) to create their respective objects,allocate local storage to such objects (if needed), and advertise theirobjects to their corresponding CMMDS sub-module 335 in order to updatethe in-memory metadata database with metadata regarding the object. Inorder to perform such requests, DOM sub-module 340 interacts with a logstructured object manager (LSOM) sub-module 350 that serves as thecomponent in VSAN module 114 that actually drives communication with thelocal SSDs and magnetic disks of its node 111. In addition to allocatinglocal storage for component objects (as well as to store other metadatasuch a policies and configurations for composite objects for which itsnode serves as coordinator, etc.), LSOM sub-module 350 additionallymonitors the flow of I/O operations to the local storage of its node111, for example, to report whether a storage resource is congested.

FIG. 3 also depicts a reliable datagram transport (RDT) sub-module 345that delivers datagrams of arbitrary size between logical endpoints(e.g., nodes, objects, etc.), where the endpoints may potentially beover multiple paths. In some embodiments, the underlying transport isTCP. Alternatively, other transports such as remote direct memory access(RDMA) may be used. RDT sub-module 345 is used, for example, when DOMsub-modules 340 communicate with one another, as previously discussedabove to create objects or to handle I/O operations. In certainembodiments, RDT module 345 interacts with CMMDS module 335 to resolvethe address of logical endpoints dynamically in order to maintainup-to-date location information in the in-memory metadata database aswell as to create, remove, or reestablish connections based on linkhealth status. For example, if CMMDS module 335 reports a link asunhealthy, RDT sub-module 345 may drop the connection in favor of a linkin better condition.

As described above, objects in objects store 116 may be made availableto computing systems outside node cluster 110. For example, one or morecomputing systems may communicate with node cluster 110, for datastorage and retrieval, through a network. A computing system, accessingnode cluster 110 for data storage and retrieval, may be referred to asan initiator. Node cluster 110 may be referred to as “storage,” and anode 111 within node cluster 110 that is accessed by the initiator maybe referred to as a target. In certain aspects, both the initiator andthe target are configured with a network-storage protocol stack thatallows the initiator and the target to exchange data over an IP network.A network-storage protocol stack, as further illustrated in FIG. 4,comprises different network and storage protocol layers (e.g., SCSIlayer, iSCSI layer, TCP layer) that facilitate the exchange ofinformation between the initiator and the target. In certainembodiments, the network-storage protocol stack is implemented in thekernel space of the initiator's or target's hypervisor. For example,hypervisor 113 of a target node 111 of node cluster 110 may beconfigured with a network-storage protocol stack to receive and processread and write requests from an initiator.

To illustrate this, in one example, a workload executing on theinitiator may need access to a file or certain data stored in nodecluster 110. In such an example, the initiator generates a read request,which is converted to a SCSI command by the SCSI layer. The SCSI commandis then converted to an iSCSI command by an iSCSI layer. The iSCSIcommand is next encapsulated by a TCP/IP layer, resulting in an iSCSIcommand with TCP/IP headers. For example, in the TCP/IP headers, theTCP/IP layer adds the IP address of the initiator as the source IPaddress and the destination IP address of the target as the destinationIP address. A user configures a target to be available to the initiatorand also configures the initiator to communicate with the target whenrequesting access to information from node cluster 110. The iSCSIcommand, comprising the read request, is then processed by a networkinterface card (NIC) driver of a NIC associated with the initiator.Subsequently, the iSCSI command is transmitted by the NIC over thenetwork to a NIC of the target in node cluster 110. An iSCSI commandthat is encapsulated, such as described above, and transmitted over thenetwork may be referred to as an iSCSI Protocol Data Unit (PDU).

The iSCSI PDU is then received at a target within node cluster 110 thatis configured with the same network-storage protocol stack as theinitiator. FIG. 4 illustrates an example network-storage protocol stack400 at the target. Network-storage protocol stack 400 comprises abackend layer 402, a SCSI layer 404, an iSCSI layer 406, a TCP/IPdatamover layer 408, a TCP/IP layer 410, and a NIC driver 412. Backendlayer 402 may refer to a set of software instructions that allow SCSIlayer 404 to interface with node cluster 110 in order to retrieve orstore information from and in the object store that is provided by nodecluster 110. SCSI layer 404 builds/receives SCSI CDBs (CommandDescriptor Blocks) and relays/receives them with the remaining commandexecute parameters to/from iSCSI Layer 406. iSCSI layer 406builds/receives iSCSI PDUs and relays/receives them to/from one or moreTCP connections that form an initiator-target “session.” TCP datamoverlayer 408 provides a set of transport primitives or operations (e.g.,connection lifecycle management and PDU transport primitives) that allowthe target to manage a connection and communicate with the initiator.TCP/IP layer 410 is configured to decapsulate iSCSI PDUs (e.g., removeTCP/IP headers) or encapsulate SCSI PDUs (e.g., append TCP/IP headers)using TCP/IP instructions.

When the iSCSI PDU, transmitted by the initiator, arrives at thetarget's NIC, it is processed by NIC driver 412, which comprises asoftware program for controlling the target's NIC. Subsequently, TCP/IPlayer 410 de-capsulates the iSCSI PDU packet by removing the TCP and IPheaders, thereby extracting the iSCSI command. The iSCSI command is thenstored in a memory location in the target's memory resources (e.g., RAMin hardware 119). This memory location is accessible by some of theupper layers, including the TCP/IP datamover layer 408, the iSCSI layer406, and the SCSI layer 404. As such, each of those upper layers is ableto further process and/or de-capsulate the iSCSI command by accessingthe iSCSI command at the same memory location. For example, iSCSI layer406 is able to access the iSCSI command at the memory location andretrieve the SCSI command.

When SCSI layer 404 accesses the SCSI command, it allocates a scattergather list (“sglist”) for the retrieval of the information that isrequested by the read request associated with the SCSI command. Thesglist is a data structure allocated in memory, with a certain startingmemory address and an ending memory address. Backend layer 402 thenpasses the read request to the node cluster 110 (e.g., VSAN module 114of the target), which processes the read request by retrieving therequested information from object store 116 and then stores theinformation in the sglist. Once the information is stored in the sglist,backend layer 402 then passes the ownership of the sglist to SCSI layer404, converts the information to a SCSI DATA-IN PDU. iSCSI layer 406then accesses the SCSI DATA-IN PDU in the sglist and converts the SCSIDATA-IN PDU into a iSCSI command. iSCSI layer 406 then allocates anotherdata structure, referred to as an “mbuffer” or “mbuf,” with a startingand an ending memory address and copies the information in the sglist tothe mbuf. This is because TCP/IP datamover layer 408 only recognizes thembuf data structure. TCP/IP datamover layer 408 then provides the memoryaddress of the mbuf to the TCP/IP layer 410, which is configured toencapsulate the iSCSI command in the mbuf to create an iSCSI PDU. Oncean iSCSI PDU is generated, TCP/IP layer 410 may copy the iSCSI PDU fromthe mbuf to buffers of NIC driver 412. Buffers of NIC driver 412 act asqueues where outgoing PDUs are stored before being transmitted over thenetwork.

Because of the two memory copies discussed above, using the TCP-basedprotocol layers (TCP/IP datamover layer 408 and TCP/IP layer 410) mayresult in latency as well as an inefficient use of compute resources.Latency is increased due to a network bottleneck associated with havingto perform memory copies for each one of a large number of read/writerequests to the target. In addition, additional compute cycles have tobe utilized for performing such memory copies.

Although FIG. 4 shows memory copies associated with a read commandreceived from an initiator, a memory copy also occurs with respect to awrite command received from the initiator. For example, initiator maygenerate a write command, convert it to a SCSI command, convert the SCSIcommand to an iSCSI command, encapsulate the iSCSI command with TCP/IPheaders, and transmit the iSCSI command with TCP/IP headers to thetarget. When the iSCSI command with TCP/IP headers is received by thetarget, the target allocates an mbuf for storing the data that will bereceived from the initiator later on. The target then sends an R2T(ready to transfer) PDU back to the initiator, indicating that thetarget is prepared for accepting any incoming DAT-OUT PDUs, that referto PDUs comprising the data that the initiator intends to send to thetarget. Once the initiator receives the R2T PDU, it transmits DATA-OUTPDUs to the target. Once the target receives a DATA-OUT PDU, TCP/IPdatamover layer 408 stores the data therein in the mbuf. The ownershipof the mbuf is then passed through the upper layers, until it reachesthe backend layer 402. However, because VSAN module 114 of the targetdoes not accept or recognize the mbuf, a memory copy has to be performedto move the data included in the mbuf to a data structure (e.g., sglist)that is recognized by node cluster 110.

Accordingly certain embodiments described herein relate to using theiSCSI Extension for Remote Direct Memory Access (RDMA) (iSER), which isa protocol designed to utilize RDMA to accelerate iSCSI data transfer.The iSER protocol is implemented as an iSER datamover layer that acts asan interface between the iSCSI layer and an RDMA layer. In other words,iSER provides the RDMA data transfer capability to the iSCSI layer bylayering iSCSI on top of an RDMA-Capable Protocol. Using iSER inconjunction with RDMA allows for bypassing the TCP/IP protocol layersand permits data to be transferred directly, between an initiator and atarget, using certain memory buffers, thereby avoiding the memory copiesdescribed above.

RDMA enables low latency transfer of information between the initiatorand the target at the memory-to-memory level, without burdening the CPUsat either the initiator or the target. This transfer function isoffloaded to the RDMA-enabled NIC (also referred to as “RNIC”) in orderto bypass the operating system's network stack (e.g., TCP/IP protocollayer). With RDMA, RNICs can work directly with the memory ofapplications, allowing data transfers over the network without the needto involve the CPU, thereby providing a more efficient and faster way tomove data between the initiator and the target at lower latency and CPUutilization.

FIG. 5 illustrates an example network-storage protocol stack 500 at thetarget. Network-storage protocol stack 500 includes backend layer 402,SCSI layer 404, iSCSI layer 406, TCP/IP datamover layer 408, iSERdatamover layer 508, TCP/IP layer 410, and RDMA layer 510. iSERdatamover layer 508 functions similar to TCP/IP datamover layer 408 andprovides the same transport primitives. iSER datamover layer 508implements the iSER protocol by providing connection lifecyclemanagement and PDU transport primitives to iSCSI layer 404, therebyallowing the transfer of iSCSI PDUs through the use of RDMA layer 510.

In the example of FIG. 5, because network-storage protocol stack 500 isconfigured with both TCP/IP datamover layer 408 and iSER datamover layer508, the target may utilize any one of the two protocols for datacommunication. For example, a user may configure the target such thatthe iSCSI layer 406 may utilize the TCP/IP datamover layer 408 to accessthe TCP/IP layer 410 when the target is configured with a standard NIC(e.g., a non-RDMA-enabled NIC) or, instead, utilize the iSER datamoverlayer 508 to access the RDMA layer 510 when the target is configuredwith an RNIC. In the example of FIG. 5, the user configures the targetwith user configuration 514, which is consumed by datamover engine 516.Datamover engine 516 refers to a set of instructions that are used toinitialize a datamover layer and establish a link between theinitialized datamover layer and iSCSI layer 406.

FIG. 6 illustrates an example flow diagram of how connection lifecyclemay be managed between an iSER target 604 and an iSER initiator 602 thatare both configured with a network-storage protocol stack having an iSERdatamover layer 508 and RDMA layer 510. An example of such anetwork-storage protocol stack was shown in FIG. 5. An iSER targetrefers to a target that has been configured with the iSER protocol(e.g., includes the iSER datamover layer in its network-storage stack).An iSER initiator refers to an initiator that has been configured withthe iSER protocol (e.g., includes the iSER datamover layer in itsnetwork-storage stack).

At step 612, iSER initiator 602 transmits a connection request to iSERtarget 604.

At step 614, upon receiving the connection request, iSER target 604 setsup an RDMA queue pair for incoming transport requests. Setting up theRDMA queue pair includes allocating a memory region in the memory of theiSER target, with a starting and an ending address, for operationsassociated with the RDMA communication between iSER initiator 602 andiSER target 604. The RDMA communication is based on a set of threequeues including a send queue, a receive queue, and a completion queue,which are all instantiated in the allocated memory region. The send andreceive queues are responsible for scheduling work and are created inpairs, also referred to as the queue pair and may be referred to as workqueues. Work queues are allocated in the allocated memory region andhold instructions as to what data (e.g., messages) stored in buffers(e.g., buffers allocated in memory storing outgoing/incoming messages)are to be sent or received. Such instructions are small structs (e.g.,composite data types) and are called work requests or work queueelements (WQE). A WQE includes a pointer to a buffer. For example, a WQEplaced on the send queue contains a pointer to a buffer address storinga message to be sent. In another example, a pointer in the WQE on thereceive queue contains a pointer to a buffer address for a location inthe buffer where an incoming message from the network can be placed. Thecompletion queue is configured to generate a notification when theinstructions placed in the work queues have been completed.

At step 616, iSER target 604 allocates a login buffer. A login buffermay also be allocated in the memory region and is configured to storeinformation (e.g., credentials) received from iSER initiator 602 forlogging in.

At step 618, iSER target 604 accepts the connection request transmittedby iSER initiator 602.

At step 620, iSER initiator 602 logs in. For example, iSER initiator 602transmits information to iSER target 604, which is stored in the loginbuffer.

At step 622, iSER target 604 then accesses the information toauthenticate and negotiate with iSER initiator 602. In one example, thenegotiation includes determining the maximum number of outstanding iSCSIcontrol-type PDUs that iSER target 604 may hold. Note that iSCSI PDUsthat cause the SCSI data to be moved between iSER initiator 602 and iSERtarget 604 may be referred to as “iSCSI data-type PDUs.” All otherpossible iSCSI PDUs may be referred to as “iSCSI control-type PDUs.”

At step 624, iSER target 604 allocates multiple memory chunks to storethe incoming outstanding iSCSI PDUs. For example, iSER target 604allocates iSCSI control-type PDU receive buffers.

At step 626, iSER target 604 transmits an indication to iSER initiator602 that indicates to iSER initiator 602 that the login has beensuccessful. Steps 620 through 626 are performed as part of a phase thatis referred to as the login phase. Upon the completion of this phase,iSER target 604 is able to fully perform iSCSI functions such as readand write operations.

At step 628, iSER initiator 602 requests a logout. For example, afterthe completion of a read operation, iSER initiator 602 sends a logoutrequest to iSER target 604.

At step 630 iSER target 604 releases the iSCSI control-type PDU receivebuffers. In some embodiments, a logout may be the result of a connectionerror, in which case, iSER target 604 removes all the outstanding I/Orequests and then releases the iSCSI control-type PDU receive buffers.

FIG. 7 illustrates operations 700 performed by network-storage stack 500at an iSER target for processing an incoming I/O read request in theform of an iSER packet, from an iSER initiator. Network-storage stack500 and the flow path of the incoming I/O request are shown in FIG. 8.Although network-storage stack 500 may, in certain embodiments, alsocomprise a TCP/IP datamover layer as well as a TCP/IP layer, in theexample of FIG. 8, those layers are not shown. Operations 700 aredescribed by reference to network-storage stack 500 of FIG. 8.

At block 702, the network-storage stack of the iSER target receives aniSER packet. For example, network-storage stack 500 receives an incomingiSER packet. An iSER packet, in some embodiments, may include an iSERheader that encapsulates an iSCSI PDU. The iSER header may indicate anidentifier (referred to as “STag”) of a remote I/O buffer at the iSERinitiator with an RNIC. The identifier informs the iSER target that theremote I/O buffer is available at the iSER initiator for RDMA read orRDMA write access by the iSER target. This remote I/O buffer is wherethe results of a SCSI read operation may be directly stored in. If theiSER packet includes a write command, the remote I/O buffer is wheredata associated with the iSCSI write operation may be retrieved from.For example, when an iSER initiator transmits a SCSI read command to aniSER target, the iSER target retrieves the requested data (i.e., resultsof the SCSI read operation) and transmits the requested data to theremote I/O buffer at the iSER initiator. More specifically, the iSERtarget writes the requested data to the remote I/O buffer using RDMAlayer 510 through an RDMA write operation.

For a SCSI write operation, the remote I/O buffer identified by the iSERheader contains the data that is to be written to the node cluster 110.For example, when an iSER initiator transmits a SCSI write command to aniSER target, the iSER target accesses the data stored in the remote I/Obuffer and retrieves the data that is stored therein. More specifically,the iSER target reads the data stored in the remote I/O buffer usingRDMA layer 510 through an RDMA read operation. In the example ofoperations 700, the iSER packet comprises a SCSI read command. In suchan example, the iSER packet has an iSER header that identifies a remoteI/O buffer where the results of the SCSI read operation will be storedat the iSER initiator.

At block 704, the network-storage stack of the iSER target decapsulatesthe iSER packet to access an iSCSI PDU. For example, whennetwork-storage stack 500 receives the iSER packet, RDMA layer 510processes the iSER packet and passes it to iSER datamover layer 508,which processes the iSER header of the iSER packet and decapsulates theiSER packet by removing the iSER header. Upon processing the iSERheader, iSER datamover layer 508 identifies the remote I/O buffer as thelocation for storing the data that is going to be retrieved from nodecluster 110 as a result of the SCSI read operation. Decapsulating theiSER packet results in an iSCSI PDU that comprises the SCSI readcommand. iSER datamover layer 508 passes the iSCSI PDU to iSCSI layer406.

At block 706, the network-storage stack of the iSER target decapsulatesthe iSCSI PDU to access a SCSI command in the iSCSI PDU. For example,iSCSI layer 406 decapsulates the iSCSI PDU received from iSER datamoverlayer 508 to access a SCSI read command.

At block 708, the network-storage stack of the iSER target generates aSCSI command structure and places the SCSI command structure in the SCSIlayer's outstanding I/O queue. For example, iSCSI layer 406 generates aSCSI command structure based on the SCSI read request and pushes theSCSI command structure to the SCSI layer 404's outstanding I/O queue.

At block 710, the network-storage stack of the iSER target translatesthe SCSI command to an I/O operation and pushes the I/O operation to anI/O queue of the backend layer. For example, SCSI layer 404 translatesthe SCSI read command to a read operation and pushes the read operationto an I/O queue of backend layer 402.

At block 712, the network-storage stack of the iSER target allocatesmemory at the iSER target's memory to hold data retrieved as a result ofthe I/O operation. For example, SCSI layer 404 allocates a scattergather list (sglist) for holding the data. As discussed, scatter-gatheris a type of memory addressing used to do direct memory access (DMA)data transfers of data that is written to noncontiguous areas of memory.A sglist is a list of vectors, each of which gives the location andlength of one segment in the overall read or write request.

At block 714, the network-storage stack of the iSER target processes theI/O operation and stores the resulting data in the memory locationallocated at step 712 (e.g., the sglist). Backend layer 402 has severalthreads that work to process I/O requests that are placed in the I/Oqueue of the backend layer 402. For example, a thread processes the readrequest pushed by SCSI layer 404 to the I/O queue of backed layer 402.Another thread may then pass the read request to the VSAN module (VSANmodule 114) of the iSER target to retrieve data requested by the readrequest. As described above, VSAN module 114 comprises a DOM sub-module340 that handles I/O operations. For example, DOM sub-module 340 handlesa read request by accessing object store 116 and retrieving the datarequested by the read request. SCSI layer 404 also passes the sglist tobackend layer 402, which in turn passes the sglist to VSAN module 114 tostore the retrieved data in the sglist. In certain embodiments, passingthe sglist to backend layer 402 may include indicating the starting andending memory addresses of the sglist. In certain embodiments, passingthe sglist to backend layer 402 may also include assigning the ownershipof the sglist to backend layer 402.

Once the read request is processed, VSAN module 114 stores the resultingdata in the sglist. Backend layer 402 then passes the ownership of thesglist, which at this points stores the resulting data, to SCSI layer404. SCSI layer 404 then accesses the data in the sglist and creates aSCSI DATA-IN PDU, comprising the data, by, for example, adding anynecessary encapsulation data. The SCSI DATA-IN PDU is stored in thesglist. SCSI layer 404 then notifies iSCSI layer about the sglist'smemory location.

At block 716, the network-storage stack of the iSER target generates aniSCSI PDU comprising the data. For example, iSCSI layer 406 accesses theSCSI DATA-IN PDU in the sglist and generates an iSCSI PDU comprising theSCSI DATA-IN PDU, which itself comprises the data resulting from theprocessing of the read request. The iSCSI layer 406 creates the iSCSIPDU by, for example, adding any necessary encapsulation information tothe SCSI DATA-IN PDU that is stored in the sglist. The iSCSI PDU isstored in the sglist. iSCSI layer 406 then notifies iSER layer 406 ofthe memory location (e.g., starting and ending memory addresses) ofsglist. Upon passing over the iSCSI PDU to iSER layer 406, iSER layer406 becomes the owner of the iSCSI PDU or the data therein.

At block 718, the network-storage stack of the iSER target generates aniSER packet using the iSCSI PDU. For example, iSER layer 406encapsulates the iSCSI PDU with an iSER header in the sglist by, forexample, adding the iSER header to the iSCSI PDU. The iSER headercomprises the identifier of the remote I/O buffer. Subsequently, iSERlayer 406 notifies iSER datamover layer 508 of the memory location ofthe sglist. iSER datamover layer 508 then communicates with RMDA layer510 to send out the iSER packet as a RDMA write operation.

At block 720, the network-storage stack of the iSER target transmits theiSER packet to the iSER initiator. For example, RDMA layer 510 transmitsthe iSER packet, including a RDMA write operation, to the RDMA layer ofthe iSER initiator. The network-storage stack of the iSER initiatorreceives the iSER packet, accesses the data within the iSER packet, andstores the data in the remote buffer.

FIG. 9 illustrates operations 900 performed by network-storage stack 500at an iSER target for processing an incoming I/O write request in theform of an iSER packet, from an iSER initiator. At block 902, thenetwork-storage stack of the iSER target receives an iSER packet. Block902 is similar to block 702 of FIG. 7, with the exception that iSERpacket in operations 900 comprises an SCSI write command. The iSERheader of the iSER packet includes a remote key and a remote I/O buffer,which stores the data that the iSER initiator intends to write to nodecluster 110.

At block 904, the network-storage stack of the iSER target decapsulatesthe iSER packet to access an iSCSI PDU. For example, whennetwork-storage stack 500 receives the iSER packet, RDMA layer 510processes the iSER packet and passes it to iSER datamover layer 508,which processes the iSER header of the iSER packet and decapsulates theiSER packet by removing the iSER header. Upon processing the iSERheader, iSER datamover layer 508 identifies the remote I/O buffer offsetassociated with a remote I/O buffer, which includes data that theinitiator intends to write to node cluster 110. iSER datamover layer 508also stores the remote key and remote I/O buffer in memory.Decapsulating the iSER packet results in an iSCSI PDU that comprises theSCSI write command. iSER datamover layer 508 passes the iSCSI PDU toiSCSI layer 406.

At block 906, the network-storage stack of the iSER target decapsulatesthe iSCSI PDU to access a SCSI command in the iSCSI PDU. For example,iSCSI layer 406 decapsulates the iSCSI PDU received from iSER datamoverlayer 508 to access a SCSI write command.

At block 908, the network-storage stack of the iSER target allocates adata structure in memory for storing data associated with the SCSI writecommand and transmits an R2T PDU to the iSER initiator to indicate thatthe iSER target is ready to receive the data. For example, iSCSI layer406 decapsulates the iSCSI PDU received from iSER datamover layer 508 toaccess a SCSI write command. When the SCSI write command reaches SCSIlayer 404, SCSI layer 404 allocates a sglist in memory for storing thedata. SCSI layer 404 then indicates to iSCSI layer 406 that the iSERtarget is now ready to receive the data. ISCSI layer 406 then transmitsan R2T PDU to the iSER datamover layer 508, which iSER datamover layer508 translates into an RDMA read operation. ISER datamover layer 508then transmits the R2T PDU to the iSER initiator. ISER datamover layer508 also feeds the remote key and remote I/O buffer offset to RDMA layer510.

At block 910, the network-storage stack of the iSER target performs anRDMA read operation to read data from the iSER initiator and store it inthe allocated memory. For example RDMA layer 510 performs an RDMA readoperation to read data that is stored in the remote I/O buffer at theiSER initiator using the remote key and the remote I/O buffer offset.The data is then stored by RDMA layer 510 in the sglist. At this time,iSER datamover layer 508 notifies iSCSI layer 404 that the data isstored in the allocated memory and it is ready for a write operationrequested by the SCSI write command (ready to be stored in node cluster110).

At block 912, the network-storage stack of the iSER target causes awrite operation associated with the SCSI write command to be performedusing the data stored in the allocated data structure. For example,iSCSI 404 passes the ownership of sglist, including the data, to backendlayer 402, which in turn passes the ownership of sglist to node cluster110 (e.g., VSAN module 114 of the iSER target). VSAN module 114 of theiSER target then causes the write operation to be performed by nodecluster 110. Causing the write operation to be performed by node cluster110 comprises indicating to node cluster 110, through backend layer 402,that node cluster 110 has ownership of the sglist, which includes thedata for the write operation. Node cluster 110 then performs the writeoperation by accessing the sglist and using the data. In operations 900,because a data structure that is recognized by node cluster 110 isallocated and used, no memory copies have to be performed, resulting ina more resource efficient and expeditious write operation.

Accordingly, the embodiments described herein provide a technicalsolution to a technical problem by using iSER in conjunction with RDMA,which allows for bypassing the TCP/IP protocol layers of a target or aninitiator and permits data to be transferred directly, between aninitiator and a target, using certain memory buffers, thereby avoidingthe memory copies associated with the use of the TCP/IP protocol layers.Note that although some aspects of the disclosure are described withrespect to a VM accessing a VSAN cluster, aspects can similarly be usedfor any virtual computing instance (VCI) or physical machine accessingany suitable distributed storage system (e.g., hyper-converged storage).

The various embodiments described herein may be practiced with othercomputer system configurations including hand-held devices,microprocessor systems, microprocessor-based or programmable consumerelectronics, minicomputers, mainframe computers, and the like.

One or more embodiments may be implemented as one or more computerprograms or as one or more computer program modules embodied in one ormore computer readable media. The term computer readable medium refersto any data storage device that can store data which can thereafter beinput to a computer system computer readable media may be based on anyexisting or subsequently developed technology for embodying computerprograms in a manner that enables them to be read by a computer.Examples of a computer readable medium include a hard drive, networkattached storage (NAS), read-only memory, random-access memory (e.g., aflash memory device), a CD (Compact Discs), CD-ROM, a CD-R, or a CD-RW,a DVD (Digital Versatile Disc), a magnetic tape, and other optical andnon-optical data storage devices. The computer readable medium can alsobe distributed over a network coupled computer system so that thecomputer readable code is stored and executed in a distributed fashion.

In addition, while described virtualization methods have generallyassumed that virtual machines present interfaces consistent with aparticular hardware system, the methods described may be used inconjunction with virtualizations that do not correspond directly to anyparticular hardware system. Virtualization systems in accordance withthe various embodiments, implemented as hosted embodiments, non-hostedembodiments, or as embodiments that tend to blur distinctions betweenthe two, are all envisioned. Furthermore, various virtualizationoperations may be wholly or partially implemented in hardware. Forexample, a hardware implementation may employ a look-up table formodification of storage access requests to secure non-disk data.

Many variations, modifications, additions, and improvements arepossible, regardless the degree of virtualization. The virtualizationsoftware can therefore include components of a host, console, or guestoperating system that performs virtualization functions. Pluralinstances may be provided for components, operations or structuresdescribed herein as a single instance. Finally, boundaries betweenvarious components, operations and data stores are somewhat arbitrary,and particular operations are illustrated in the context of specificillustrative configurations. Other allocations of functionality areenvisioned and may fall within the scope of one or more embodiments. Ingeneral, structures and functionality presented as separate componentsin exemplary configurations may be implemented as a combined structureor component. Similarly, structures and functionality presented as asingle component may be implemented as separate components. These andother variations, modifications, additions, and improvements may fallwithin the scope of the appended claims(s). In the claims, elementsand/or steps do not imply any particular order of operation, unlessexplicitly stated in the claims.

We claim:
 1. A method of processing an incoming packet by a targetdevice associated with a distributed storage system, the methodcomprising: receiving the incoming packet from an initiator device, theincoming packet encapsulated by the initiator device using an InternetSmall Computer Systems Interface (iSCSI) extension for remote directmemory access (RDMA) (iSER) of the initiator device; decapsulating thepacket to access a Small Computer Systems Interface (SCSI) read commandindicating an input/output (I/O) operation to perform at the distributedstorage system; allocating a data structure in a memory of the targetdevice for storing data received from the distributed storage system asa result of performance of the I/O operation; storing the data in thedata structure; encapsulating the data in the data structure to generatea SCSI DATA-IN PDU including the data, wherein the SCSI DATA-IN PDU isstored in the data structure; encapsulating the SCSI DATA-IN PDU togenerate an outgoing iSCSI protocol data unit (PDU) including the SCSIDATA-IN PDU, wherein the outgoing iSCSI PDU is stored in the datastructure; encapsulating the iSCSI PDU to generate an outgoing iSERpacket; and transmitting the iSER packet to the initiator device.
 2. Themethod of claim 1, wherein the decapsulating comprises removing an iSERheader from the incoming packet.
 3. The method of claim 1, wherein thedata structure comprises a scatter-gather list.
 4. The method of claim1, wherein encapsulating the data in the data structure to generate aSCSI DATA-IN PDU is performed by a SCSI protocol layer of anetwork-storage stack of the target device, the method furthercomprising: notifying, at the SCSI protocol layer, a iSCSI protocollayer of the network-storage stack about a memory location of the datastructure, after encapsulating the data in the data structure togenerate the SCSI DATA-IN PDU.
 5. The method of claim 1, whereinencapsulating the data in the data structure to generate a iSCSI PDU isperformed by a iSCSI protocol layer of a network-storage stack of thetarget device, the method further comprising: notifying, at the iSCSIprotocol layer, a iSER protocol layer of the network-storage stack abouta memory location of the data structure, after encapsulating the data inthe data structure to generate the iSCSI PDU.
 6. A target apparatus,comprising: a non-transitory memory comprising executable instructions;and a processor in data communication with the memory and configured toexecute the instructions to cause the apparatus to perform a method, themethod comprising: receiving the incoming packet from an initiatorapparatus, the incoming packet encapsulated by the initiator deviceusing an Internet Small Computer Systems Interface (iSCSI) extension forremote direct memory access (RDMA) (iSER) of the initiator apparatus;decapsulating the packet to access a Small Computer Systems Interface(SCSI) read command indicating an input/output (I/O) operation toperform at the distributed storage system; allocating a data structurein a memory of the target apparatus for storing data received from thedistributed storage system as a result of performance of the I/Ooperation; storing the data in the data structure; encapsulating thedata in the data structure to generate a SCSI DATA-IN PDU including thedata, wherein the SCSI DATA-IN PDU is stored in the data structure;encapsulating the SCSI DATA-IN PDU to generate an outgoing iSCSIprotocol data unit (PDU) including the SCSI DATA-IN PDU, wherein theoutgoing iSCSI PDU is stored in the data structure; encapsulating theiSCSI PDU to generate an outgoing iSER packet; and transmitting the iSERpacket to the initiator apparatus.
 7. The target apparatus of claim 6,wherein the decapsulating comprises removing an iSER header from theincoming packet.
 8. The target apparatus of claim 6, wherein the datastructure comprises a scatter-gather list.
 9. The target apparatus ofclaim 6, wherein encapsulating the data in the data structure togenerate a SCSI DATA-IN PDU is performed by a SCSI protocol layer of anetwork-storage stack of the target apparatus, wherein the methodfurther comprises: notifying, at the SCSI protocol layer, a iSCSIprotocol layer of the network-storage stack about a memory location ofthe data structure, after encapsulating the data in the data structureto generate the SCSI DATA-IN PDU.
 10. The target apparatus of claim 6,wherein encapsulating the data in the data structure to generate a iSCSIPDU is performed by a iSCSI protocol layer of a network-storage stack ofthe target device, wherein the method further comprises: notifying, atthe iSCSI protocol layer, a iSER protocol layer of the network-storagestack about a memory location of the data structure, after encapsulatingthe data in the data structure to generate the iSCSI PDU.
 11. Anon-transitory computer readable medium having instructions storedthereon that, when executed by a target device, cause the target deviceto perform a method comprising: receiving the incoming packet from aninitiator device, the incoming packet encapsulated by the initiatordevice using an Internet Small Computer Systems Interface (iSCSI)extension for remote direct memory access (RDMA) (iSER) of the initiatordevice; decapsulating the packet to access a Small Computer SystemsInterface (SCSI) read command indicating an input/output (I/O) operationto perform at the distributed storage system; allocating a datastructure in a memory of the target device for storing data receivedfrom the distributed storage system as a result of performance of theI/O operation; storing the data in the data structure; encapsulating thedata in the data structure to generate a SCSI DATA-IN PDU including thedata, wherein the SCSI DATA-IN PDU is stored in the data structure;encapsulating the SCSI DATA-IN PDU to generate an outgoing iSCSIprotocol data unit (PDU) including the SCSI DATA-IN PDU, wherein theoutgoing iSCSI PDU is stored in the data structure; encapsulating theiSCSI PDU to generate an outgoing iSER packet; and transmitting the iSERpacket to the initiator device.
 12. The non-transitory computer readablemedium of claim 11, wherein the decapsulating comprises removing an iSERheader from the incoming packet.
 13. The non-transitory computerreadable medium of claim 11, wherein the data structure comprises ascatter-gather list.
 14. The non-transitory computer readable medium ofclaim 11, wherein encapsulating the data in the data structure togenerate a SCSI DATA-IN PDU is performed by a SCSI protocol layer of anetwork-storage stack of the target apparatus, wherein the methodfurther comprises: notifying, at the SCSI protocol layer, a iSCSIprotocol layer of the network-storage stack about a memory location ofthe data structure, after encapsulating the data in the data structureto generate the SCSI DATA-IN PDU.
 15. The non-transitory computerreadable medium of claim 11, wherein encapsulating the data in the datastructure to generate a iSCSI PDU is performed by a iSCSI protocol layerof a network-storage stack of the target device, wherein the methodfurther comprises: notifying, at the iSCSI protocol layer, a iSERprotocol layer of the network-storage stack about a memory location ofthe data structure, after encapsulating the data in the data structureto generate the iSCSI PDU.
 16. A method of processing an incoming packetby a target device associated with a distributed storage system, themethod comprising: receiving the incoming packet from an initiatordevice, the incoming packet encapsulated by the initiator device usingan Internet Small Computer Systems Interface (iSCSI) extension forremote direct memory access (RDMA) (iSER) of the initiator device;identifying a remote I/O buffer offset in an iSER header of the packet,the remote I/O buffer offset being associated with a remote I/O bufferat the initiator device, the remote I/O buffer including data associatedwith a SCSI write command in the incoming packet; storing the remote I/Obuffer in a memory of the target device; decapsulating, at the iSER ofthe target device, the packet to access an Internet Small ComputerSystems Interface (iSCSI) packet data unit (PDU); decapsulating theiSCSI PDU to access the SCSI write command; allocating a data structurein the memory for storing the data associated with the SCSI writecommand; performing an RDMA operation to read the data from the remoteI/O buffer of the initiator device; storing the read data into the datastructure; and causing a write operation associated with the SCSI writecommand to be performed by the distributed storage system, wherein thedistributed storage system accesses the data structure to perform thewrite operation using the data in the data structure.